The present invention relates to a method of manufacturing traction rollers for continuously variable transmissions and traction rollers resulting therefrom.
Various types of continuously variable transmissions have been proposed, one of which is a toroidal-type continuously variable transmission comprising input and output discs and a power roller or traction roller frictionally engaged therewith for ensuring power transmission.
The method of manufacturing power rollers will be described. Conventionally, the detailed method has not been proposed except forming power rollers out of AISI52100 corresponding to JIS SUJ2, high-carbon high-chromium bearing steel or carburizing steel having an effective hardened layer of 2.0-4.0 mm as disclosed in JP-A 7-71555. Ordinarily, the power rollers can be manufactured as follows:
A solid cylindrical material having the same length as the axial length of a power roller is prepared and machined to obtain the final shape, which is subjected first to deep carburizing or carbonitriding of, e.g. 3 mm at 830-960.degree. C., then 840-890.degree. C. during 24-110 hr, and then to hardening. Subsequently, this work is tempered at 170-180.degree. C. during 2-5 hr, and subjected to grinding finishing to obtain a finished product.
However, manufacturing of the power rollers in such a way needs not only a high material cost due to bad material yield, but a lot of time for deep carburizing or carbonitriding. Additionally, a process of deep carburizing or carbonitriding cannot ensure separate treatment of the power roller outer peripheral surface and central axial hole inner peripheral surface which require a thick carburized or carbonitrided layer, and the ball receiving side wall which requires a thin carburized or carbonitrided layer. Moreover, hardened layers formed through the process remain on the finished product without any elimination. These facts can cause the following problems:
When the power roller outer peripheral surface and central axial hole inner peripheral surface, and the ball receiving side wall are treated at the same time, the thickness of the carburized or carbonitrided layer of the ball receiving side wall which may be thin is equal to that of the carburized or carbonitrided layer of the power roller outer peripheral surface and central axial hole inner peripheral surface which is to be thick. Thus, the confluence of the central axial hole inner peripheral surface and the ball receiving side wall which should be the most durable in view of the strength of the power roller is difficult to be formed with a soft core, having considerably lowered durability with respect to crack, resulting in a harmful influence on the durability of the power roller.
Further, due to the fact that any elimination of the hardened layers is not carried out, austenite grains grown through the above process remain on the finished product, resulting in inevitable lowering of the toughness of the hardened layers.
Still further, since machining causes disconnection of a fiber or fabric flow of the solid cylindrical material at the power roller outer peripheral surface and the ball receiving side wall which serve as rolling surfaces of the input and output discs and a ball or rolling element, respectively, the disconnected portion of the fiber flow is apt to be a start point of breakaway and fatigue or impact crack. According to the above method, this problem is aggravated when the austenite grain size is large, i.e. the grading number is 7 or less.
Furthermore, due to the necessity of increasing a duration of carburizing or carbonitriding for the reason as described above, an intergranular oxidizing zone of the confluence of the central axial hole inner peripheral surface and the ball receiving side wall which often causes a crack has a relatively large thickness, which cannot completely be eliminated through the above grinding finishing.
It is, therefore, an object of the present invention to provide a method of manufacturing traction rollers for continuously variable transmissions which contributes to an improvement of the durability of traction rollers.